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by Ron Kurtus (revised 14 November 2014)

Temperature is a measure of how hot or cold an object or system is. It is approximately equal to the average thermal kinetic energy of an object's atoms or molecules.

This measurement of temperature is done with a device called a thermometer, which gives temperature values relative to a defined temperature. Common temperature scales are Celsius, Fahrenheit, and Kelvin.

The lower limit of temperature is when the kinetic energy of the particles approaches zero. This temperature is called absolute zero. The upper temperature limit is determined by the maximum possible velocity of the particles.

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This lesson will answer those questions. Useful tool: Units Conversion

Temperature is average kinetic energy

The thermal energy of an object consists of the total kinetic energy of all its atoms and molecules. Since the kinetic energies of individual atoms or molecules in the object vary over some range, their average is designated as the temperature of the object. This means that the temperature should be independent of the size of the object.

The kinetic energy of an object's particles is dependent on the phase or state of the material. With an ideal gas, temperature can be related to the velocity of the particles. However, in most gases, liquids and gases, the motion of the particles is more complex and the relationship does not directly hold.

Measurement of temperature

Temperature is measured with a thermometer. This is a device that records physical changes in a material when subjected to varying temperatures. The most common thermometer measures the expansion of a liquid with the change in temperature.

Thermal energy is transferred from an object to the thermometer until they are both at the same temperature. This is the determination of the object's temperature.

Temperature scales

A scale is added to the thermometer, in order to give readings of the relative temperature. In the Celsius scale, the freezing point of pure water is set at 0° C (zero degrees Celsius), and the boiling point of water is set at 100° C. The scale is divided into 100 units of temperature, called degrees.

Other temperature scales are the Fahrenheit, Kelvin and Rankine scales.

(See Temperature Measurement: Thermometers for more information on this subject.)

Temperature limits

There are upper and lower limits to temperature.

Upper limit

The highest possible temperature is limited by the greatest speed possible for a particle of matter, which is the speed of light. According to the Theory of Relativity, matter can approach that speed but never reach it.

The thermal energy is determined by Einstein's famous E = mc² equation.

Lower limit

The lowest or coldest temperature possible is called Absolute Zero. This absolute zero limit is the temperature of an object when all of its matter has zero energy.

Quantum Mechanics laws state that Absolute Zero can be approached but never reached, because matter cannot have zero energy.

(See Temperature Limits for more information on this subject.)


Temperature is the average intensity of the thermal energy of an object. The lower limit of temperature is called absolute zero. The upper temperature limit is determined by the speed of light. Temperature is measured with a device called a thermometer that gives relative readings.

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